refactor: _standard_fit method made redundant.

[fjosw]

I started refactoring the fit code and removed the _standard_fit function by merging its functionality with _combined_fit. All tests pass so it looks like nothing went wrong in the process but please have a close look @s-kuberski & @PiaLJP .

[s-kuberski]

Thank you for the changes. I spotted a bug that is not caught by the tests.

[s-kuberski]

Thanks! The two functions residual_plot and qqplot are invalid for correlated fits. This has been the case before and I don't say that we need to resolve this issue now. If, at some point, we want to show the transformed residuals that enter a correlated fit, we would have to supply chol_inv and the complete x and y vectors, right?

[fjosw]

I would say that residual_plot is not invalid, one just would not expect that the residuals are distributed ~ N(0, 1) if there are correlations between the data points. I think one could transform the residuals, but I am unsure if the transformation and the resulting visual representation are unambiguous. I would propose leaving it as it is for now.

[s-kuberski]

I agree. We can keep this in mind for the future.

[PiaLJP]

Maybe it would be a good idea to add a warning then, along the lines of 'For correlated data points the residuals are not necessarily expected to be ~N(0,1)'? Apart from that I also think it looks good now, thanks.